Apparatus for electro-fishing

ABSTRACT

Electro-fishing apparatus comprising a trawling net (16) which during operation is towed over the bottom, a combination of electrodes (10, 12) which, during operation, are towed over the bottom at a distance in front (14) of the net (16), and a pulse generator (28) which is connected to electrodes (10, 12) to generate during operation a pulsed electric field in the water. The electrodes comprise at least partly a number of electrically conducting bodies (10a, 10b, 10c, . . . ) each through a separate connection cable connected to pulse generator (28), whereby the water contacting surface of each electrically conducting body (10) is limited such that a specific electrical resistance is created. The pulse generator (28) is embodied such that each of electrically conducting bodies (10) can be powered separately to generate a field between the momentaneously powered electrically conducting body (10) and another electrode (12).

FIELD OF THE INVENTION

The invention relates to an apparatus for electro-fishing comprising

a trawl net which during operation is towed over the bottom,

a combination of electrodes which, during operation, are towed over thebottom at a distance in front of the net,

a pulse generator which is connected to said electrodes to generateduring operation a pulsed electric field in the water.

BACKGROUND OF THE INVENTION

Apparatuses for electro-fishing of the above-described type are alreadyknown from various patent publications. The attention is drawn forexample to U.S. Pat Nos. 3,483,649, 3,491,474, 3,110,978, 3,777,388,4,417,301, 5,111,379, and 5,233,782.

In the apparatuses described in U.S. Pat Nos. 3,483,649 and 3,491,474,the electrodes are embodied as a number of not insulated electricalconductors which are positioned in front of the trawl net over the fullwidth thereof and mutually parallel.

The electrodes which are applied in the apparatuses described in U.S.Pat. Nos. 3,777,388 and 4,417,301 consist of a number of elongated notinsulated electrical conductors, which, mutually parallel, extend in thetrawling direction, i.e. perpendicular to the front edge of the trawlnet. Also in this case all anode conductors and all cathode conductorsare powered simultaneously by the pulse generator.

The use of relatively long, mutually parallel extending non insulatedelectrical conductors as electrodes has a number of disadvantages. Inthe first place sea water is well-conducting. It may be expectedtherefore, that the total impedance, which in such an embodiment of theelectrodes is connected to the pulse generator, is relatively small withthe result that the pulse generator has to produce relatively largecurrent pulses to realize the desired field gradient between theelectrodes. A further disadvantage is encountered in the fact that thefield strength varies along the length of the electrical conductors,near the pulse generator the field strength will be high and as thedistance increases the field strength will decrease. Therefore, a fieldwith the same strength will not be generated everywhere and as a resultthereof the fish, which are on or in the bottom, will not everywherereceive the same stimulation.

The electrodes which belong to the apparatus as described in U.S. Pat.No. 3,110,978 comprise one or more anode-components and one or morecathode-components. Irrespective of the number of elements allelectrodes are simultaneously powered from the pulse generator. In thispublication no details are provided about the embodiment of theelectrodes themselves. The use of separate electrode components in thisapparatus has not resulted in a decrease of the power to be supplied. Inthis publication pulses with a peak voltage of 700 V and a peak currentof 10,500 Amp are mentioned. To be able to switch such voltages andcurrents the switching elements in the pulse generator have to fulfilvery high requirements.

In the U.S. Pat. Nos. 5,111,379 and 5,233,782 no details are providedabout the electrodes, although an embodiment of the pulse generator isconsidered in more depth. Especially a pulse generator is describeddestined to supply pulse trains with a number of relatively highfrequency pulses, which pulse trains are provided in a relatively lowfrequent pace to the electrodes. Furthermore, these publicationsconsider in more depth the relation between the strength of thegenerated electric field and the influence thereof on the fish arrivingin said field. Not only the field strength as such is of importance,also the steepness of the pulses, used for generating said field, hasinfluence on the reactions, evoked in the fish.

SUMMARY OF THE INVENTION

The object of the invention is now to provide an apparatus which isspecially destined for at least temporarily anaesthetizing or stunningthe fish, arriving in the generated field and using therefore a pulsegenerator with a relatively restricted power output.

In agreement with said object the invention now provides an apparatus asdescribed in the first paragraph, which is characterized in that

at least part of the electrodes comprises a number of electricallyconducting bodies, each through a separate connection cable connected tosaid pulse generator, whereby the water contacting surface of eachelectrically conducting body is limited such that a specific electricalresistance is created,

the pulse generator is embodied such that each of said electricallyconducting bodies can be powered separately to generate a field betweenthe momentaneously powered electrically conducting body and anotherelectrode.

Because the pulse generator each time supplies current pulses to oneelectrically conducting body of which the water contacting surface isrestricted and has as result thereof a certain resistance, the currentto be supplied by the pulse generator is restricted. Although a field ofrestricted extension is thereby created, by activating each time anothercombination of an electrically conducting body and a counter electrodenevertheless a large bottom area, especially a strip of ground directlyin front of the trawl net, is each time locally stimulated.

In a simple embodiment the other electrode comprises an elongatedconductor extending at a predetermined distance parallel to the firstelectrode which comprises a linear series of electrically conductingbodies. This embodiment has the advantage that the pulse generator onlyhas to switch between the electrically conducting bodies forming the oneelectrode, the other electrode embodied as an elongated conductor, canbe connected steadily to the pulse generator and does not have to beswitched. A disadvantage of this embodiment, however, is that thegenerator field fans out in the direction of the elongated conductor andpresents therefore a rather strongly varying field gradient.

A much better defined field is each time obtained in case the otherelectrode is also formed by one of the electrically conducting bodies.In that case each time a field is generated between two electricallyconducting bodies with restricted surface, the gradient of the fieldbeing well controllable. Preferably both electrodes are embodied as alinear array of electrically conducting bodies, whereby both arrays havea predetermined mutual distance.

A disadvantage of the above-described embodiments could be that thenumber of switching elements in the pulse generator is relatively largebecause each time in both arrays electrically conducting bodies have tobe switched. This disadvantage can be eliminated by comprising theapparatus with one single linear array of electrically conductingbodies, where the other electrode is each time formed by one of theelectrically conducting bodies which is adjacent in the array. In thatcase no field is generated between two electrodes which are one behindthe other in the trawling direction, but between two electrodes whichare neighbouring in the trawling direction.

The invention will be explained in more detail with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a trawl net with in front thereofpositioned electrodes according to the invention during operation.

FIG. 2 shows a cross section through a possible embodiment of the cablewith electrode segments according to the invention.

FIG. 3 shows an embodiment of the power supply and control circuitaccording to the invention.

FIGS. 4a . . . 4c illustrate alternative embodiments of the electrodes.

FIG. 5 illustrates an other embodiment of the power supply and controlcircuit.

FIGS. 6a and 6b illustrate other embodiments of the cable and theelectrode segments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment of the electrodes, which can beapplied in an apparatus according to the invention. In this figure thefirst electrode is referenced by 10, the second electrode extendingparallel thereto is referenced by 12 and both electrodes are extendingparallel to the front edge 14 of the trawl net 16. The electrodes 10, 12and the trawl net 16 are attached to doors 18 and 20. During operationthe whole combination is dragged across the bottom of the fishinggrounds in the direction of the arrow 26 through the drag cables 22 and24 by a ship which is not illustrated in the figure. The electrodes 10and 12 are connected to a pulse generator 28 which is attached to one ofthe doors 18 and which is by means of an electrical cable 30 poweredfrom a power source on board the ship such as an accumulator or agenerator. This power source is not indicated in the figure. For detailsthe attention is amongst others drawn to the above-mentioned patentpublications and to further literature, which is accessible to theaverage practitioner.

In FIG. 1 the electrode 10 is embodied as an electrode which comprises anumber of separate electrically conducting bodies 10a, 10b, 10c, . . . ,which each are separately connected through an electrical cable to thepulse generator 28 in a manner as will be explained in more detail withreference to FIG. 2. Furthermore, in this example the electrode 12 isembodied as an elongated non-insulated electrical conductor which alsois connected through a connecting cable to the pulse generator 28.

FIG. 2 illustrates a cross section through a possible embodiment of theelectrode 10. In this embodiment the electrode comprises a centralelement consisting of a well-insulated multicore cable 32 of which thejacket is referenced by 34. Within said jacket there are a number ofmutually insulated conductors 36a, 36b, . . . of which the numbercorresponds with the number of electrically conducting bodies positionedaround the central multicore cable 32. Two of said electricallyconducting bodies are in general referenced by 38a and 38b. Each of thebodies 38a, 38b, . . . is, as is shown in the figure, embodied as a ringor sleeve of conducting material 40a, 40b, . . . Each of these rings isshifted onto the cable 32 and thereafter attached by means of sealingelements 42a, 44a, 42b, 44b, These sealing elements can be formed bymeans of a pliable elastic mass, such as pliable rubber or somethingsimilar. Before fixing each of the electrode segments a passage is madein the jacket of the cable 32 at a location within the respective ring40 and through said passage one of the conductors 36 is drawn out of thecable. After clearing the end of the respective conductor such that theactual core will be revealed, this inner core is in a suitable mannerattached to the metal ring 40. In FIG. 2 this is shown for theconductors 36a and 36b. It will be clear from this figure that, by usingthe sealing elements 42 and 44, it is prevented that water willpenetrate into the tube 40 and therewith eventually could penetrate intothe cable 32.

As a result of the construction, schematically illustrated in FIG. 2,only the outer surfaces of the rings or sleeves 40a, 40b, . . . , are incontact with seawater. The dimensioning of the sleeves 40 is selectedsuch that a predetermined resistance is created which on the one hand isdependent on the surface of the sleeve 40 being in contact with seawaterand which is on the other hand mainly dependent on the salinity of theseawater. The smaller the surface of the sleeve 40, the greater theresistance; the greater the salinity of the water, the smaller theresistance. The surface of the sleeve 40 can therefore be used torestrict the current, which has to be supplied by the connected pulsegenerator, to a predetermined maximum.

FIG. 3 illustrates a possible embodiment of a pulse generator which canbe applied in the apparatus according to the invention. The pulsegenerator is connected to a power supply source 50, for instancecomprising one or more accumulators, an AC voltage generator incombination with a rectifying circuit or something similar. This powersupply source 50 supplies energy through cable 51 on the one hand to theelectrode 12 and on the other hand to a number of switching circuits52a, 52b, . . . The outputs of these switching circuits are connected tothe various conductors 36a, 36b, . . . within the cable 32. The inputsof these switching circuits are connected to an electronic controlcircuit 54 in which control pulses are generated to sequentially switchthe switching circuits each separately such that through the respectiveconductor 36 a current pulse is supplied to the respective electricallyconducting body. The components 52a, 52b, . . . and 54 are confinedwithin a watertight sealed housing which is schematically indicated bythe enclosure 28. This enclosure 28 is attached to one of the doors 18as is shown in FIG. 1. The energy source 50 is located on board of aship and a cable 30 extends from the ship to the actual generator 28. Itis possible to embody the generator 28 such that the generator canfunction automatically without external signals and will be switched onfor instance as soon as the generator will reach a predetermined depth.However, it is also possible, as is schematically illustrated in FIG. 3,to install a control unit 58 on board of the ship and to connect thiscontrol unit through a control cable 59 to the electronic controlcircuit 54 within the generator 28.

It is furthermore conceivable to locate the power source within thewatertight enclosure 28 (or eventually within a separate enclosure),such that the long power supply cable between the source 50 and thevarious switching circuits can be reduced to a short direct connection.Such an embodiment of the system, however, will only be practicalapplicable as the dimensions of the power source are small enough. Anexample of small enough power sources are the gas generators which areavailable at this moment.

Within the scope of the invention a combination of electrodes may comein various shapes, some of which are illustrated in FIG. 4. FIG. 4ashows schematically a configuration with a first electrode 60,comprising a number of separate electrically conducting bodies 62a, 62b,as described above and a second electrode 64 consisting of one elongatedconductor. If the conductor 64 is steadily connected to one terminal ofthe power supply and the electrically conducting bodies 62a, 62b, . . .are sequentially one after the other connected through the pulsegenerator, then each time a field will be generated of which the shapeis approximately schematically illustrated between the conducting body62a and the elongated conductor 64.

The pace with which the various bodies 62 are switched depends on thespeed of dragging the configuration across the bottom. If the distancebetween both electrodes 60 and 64 is equal to L and there are as a wholek electrically conducting bodies 62 present within the electrode 60 andthe ship is dragging with a speed v, then the switching frequency shouldbe at least k.v/L. Furthermore, the mutual distance between the variousconducting bodies 62a, 62b, . . . should be selected such, that the eachtime generated fields are at least partly overlapping.

The fields generated each time between each of the bodies 62 and theconductor 64 show a significant fan out near the conductor 64,indicating a significant variation in the field strength over thedistance L. A more defined situation can be obtained in case theconfiguration of FIG. 4b is applied. In this embodiment also theelectrode 64 is embodied as an electrode comprising separate conductingbodies 66a, 66b, . . . By switching these separate bodies 66a, 66bsynchronously with the bodies 62a, 62b, . . . each time a well definedfield is generated of which the shape is schematically shown between thebodies 62a and 66a. The fan out of the field and therewith the variationin field strength is in this case less significant and therefore thisconfiguration is suitable to generate an accurately quantified field ofsufficient strength to stimulate and anaesthetize the fish, present inthe bottom, without generating locally high intensities causingelectrocution of the fish. It is the object to get the fish alive andwell within the trawl net. It is furthermore preferred in thisembodiment that the surfaces of the separate electrode segments 62a,66a, . . . are equal to minimize therewith the variation in fieldstrength and to at least define said variation enabling by a properselection of voltage and current to generate the desired field strengthwith the desired anaesthetic effect.

A disadvantage of the configuration in FIG. 4b is found in the fact thatthe number of power switches 52n should be doubled in relation to theconfiguration in FIG. 4a, because the number of separately switchableelectrically conducting bodies is doubled. This disadvantage can beeliminated with the embodiment which is illustrated in FIG. 4c. In thisembodiment only one array of electrically conducting bodies is usedwhich preferably, as schematically indicated, have a relatively largediameter. The pulse generator to be used with this configuration isadapted in a suitable manner such, that initially the electrode body 62ais connected with the one terminal of the power supply source 50 whereassimultaneously the neighbouring electrode body 62b is connected to theother terminal of the power supply source 50. Therewith a field isgenerated between both bodies the shape of which is schematicallyindicated in FIG. 4b. Subsequently, by correct switching a field isgenerated between the electrode body 62b and the neighbouring electrodebody 62c, etc. The clear advantage of this embodiment is that only oneelectrode 60 is needed, which, however, implies that a rathercomplicated pulse generator has to be used of which an embodiment isschematically illustrated in FIG. 5.

The pulse generator from FIG. 5 comprises the energy source 70, whichthrough the cable 71 is connected to the actual pulse generator 76. Bothterminals of the cable 71 are in this case connected to one of the powerswitches 72a, 72b, . . . The control input of each of these powerswitches 72 is connected to the electronic control circuit 74. Eachpower switch 72 can be considered as a three-position switch connectingthe output to either the one terminal of the power supply source, or theother terminal of the power supply source, or with none of theterminals. It will be clear that the control signals supplied by thegenerator 76 should be adapted to this embodiment of the power supplyswitch 72. It is in this respect, however, pointed out that thepractical realization of a suitably functioning pulse generator afterreading the above paragraphs is considered within reach of the averagepractitioner, so that the provision of further details is consideredsuperfluous.

It is remarked that above the separate electrode bodies are indicated asrings or sleeves of electrically conducting material, attached around amulticore cable with smaller diameter than the diameter of the rings orsleeves. It will be clear to the average practitioner that the diameterof the rings or sleeves and the diameter of the cable can be selectedsuch that the rings are closely fit around the outer jacket of themulticore cable, so that eventual problems in relation to the sealingbetween the rings and the cables are decreased and furthermore the cableas a whole obtains a more smooth shape, so that the cable can be draggedwith less mechanical resistance across the sea bottom.

Instead of the rings or sleeves also flat platelets can be used whichfor instance by means of an adhesive or something else are attached tothe outer jacket of the cable. An example thereof is schematicallyillustrated in FIG. 6a. The outer jacket of the cable is in this figurereferences by 77 and within the jacket there are the mutually insulatedconductors 78a, 78b, . . . On the outer jacket of the cable an electrodebody shaped as an elongated rectangular flat platelet 79 of electricallyconducting material is adhered or glued using a suitable adhesive orglue. Such adhesives are as such known and the average practitioner doesnot need further details. It will be understood that, preceding theapplication of adhesive or glue one of the conductors 78a, 78b, . . . isguided through the jacket 77 of the cable outside and is in anelectrically conductive manner attached to the platelet 79 in a similarmanner as schematically illustrated in FIG. 2.

It is furthermore conceivable to apply, instead of a multicore cablewith an approximately circular cross section of the outer jacket, a bandor ribbon cable in which the various cores are more or less positionedbeside each other. In that case it is preferred to embody theelectrically conducting bodies not as a ring or sleeve, as discussedabove, but to embody them as a flat platelet, which in a suitable manneris attached to the ribbon cable. A conceivable practical embodiment isschematically illustrated in FIG. 6a. The cable comprises a number ofelectrical conductors 81a, 81b, . . . , which extend in general parallelin the same plane and are positioned within and surround by aninsulating jacket 80. Band cables of this type are known as such. As isillustrated in FIG. 6b a flat plate 82 of electrically conductingmaterial is attached to the cable by means of a mounting element 83,which on the one hand adheres very well to the plate 82 and on the otherhand adheres very well to the jacket 80 of the multicore cable. Withcables of this type it is in general possible to free one of the cores81 out of the cable without obstructing the insulation of the othercores. That implies that the use of such a band cable minimizes eventualproblems in relation to penetration of fluid in the cable.

I claim:
 1. Electro-fishing apparatus comprising:a trawl net, aplurality of electrodes in front of the net when the net is being towed,a pulse generator supplying pulses to said plurality of electrodes togenerate a pulsed electric field, a power source providing electricalpower to the pulse generator, each electrode connected through aseparate insulated connection cable to said pulse generator, switchingmeans having an input and a plurality of outputs, the input connected tothe pulse generator and each output connected to one of said insulatedconnection cables, a control circuit controlling the switching means sothat only two of said electrodes at a time are powered by the pulsegenerator whereby an electric field is generated only between said twoelectrodes.
 2. Electro-fishing apparatus according to claim 1, whereinat least part of the electrode consists of an electrically conductingbody with a water contacting surface having a water salinity-specificelectrical resistance.
 3. Electro-fishing apparatus according to claim1, wherein the control means control the switching means such thatsequential order electric fields are generated between two differentelectrodes or electrode bodies respectively.
 4. Electro-fishingapparatus according to claim 2, wherein the control means control theswitching means such that sequential order electric fields are generatedbetween two different electrodes or electrode bodies respectively. 5.Electro-fishing apparatus comprising:a trawl net, a plurality ofelectrodes in front of the net when the net is being towed, a pulsegenerator supplying pulses to said plurality of electrodes to generate apulsed electric field, a power source providing electrical power to thepulse generator, at least part of the electrodes consisting ofelectrically conducting bodies, each of said electrically conductingbodies having a water contacting surface having water salinity-specificelectrical resistance, the other part of the electrodes consisting of atleast an elongated conductor which through a further insulated conductoris connected to the pulse generator, said electrically conducting bodiesconnected through a separate insulated connection cable to said pulsegenerator, switching means having an input and a plurality of outputs,the input connected to the pulse generator and each output connected toone of said insulated connection cables, a control circuit controllingthe switching means wherein through its insulated connection cable onlyone conducting body at a time is powered by the pulse generator togetherwith said elongated conductor whereby an electric field is generatedonly between said only one conducting body and the elongated conductor.6. Electro-fishing apparatus comprising:a trawl net, a plurality ofelectrodes in front of the net when the net is being towed, a pulsegenerator supplying pulses to said plurality of electrodes to generate apulsed electric field, a power source providing electrical power to thepulse generator, at least part of the electrodes consisting of a firstset of electrically conducting bodies with a water contacting surfacehaving a water salinity-specific electrical resistance, the other partof the electrodes consisting of a second set of conducting bodies with awater contacting surface having a water salinity-specific electricalresistance, each of the electrically conducting bodies connected througha separate insulated connection cable to said pulse generator, switchingmeans having an input and a plurality of outputs, the input connected tothe pulse generator and each output connected to one of said insulatedconnection cables, a control circuit controlling the switching meanswherein through its insulated connection cable only at a time only oneconducting body of the first set and only one conducting body of thesecond set are powered by the pulse generator whereby an electric fieldis generated only between said two conducting bodies.
 7. Electro-fishingapparatus comprising:a trawl net, a plurality of electrodes in front ofthe net when the net is being towed, a pulse generator supplying pulsesto said plurality of electrodes to generate a pulsed electric field, apower source providing electrical power to the pulse generator, at leastone of the electrodes consisting of electrically conducting bodies, eachof said electrically conducting bodies having a water contacting surfacehaving a water salinity-specific electrical resistance, the other partof the electrodes consisting of an electrically conducting body with awater contacting surface having a water salinity-specific electricalresistance, each of said electrically conducting bodies connectedthrough a separate insulated connection cable to said pulse generator,switching means having an input and a plurality of outputs, the inputconnected to the pulse generator and each output connected to one ofsaid insulated connection cables, a control circuit controlling theswitching means wherein through its insulated connection cable only twoconducting bodies at a time are powered by the pulse generator wherebyan electric field is generated only between said two conducting bodies.